This invention relates to a synthesizer for complex multiphase waveforms, and more particularly to a digital system for producing a plurality of binary (high and low level) waveforms wherein the intervals during which a particular waveform is high may be set independent of other waveforms, and the extent of the intervals themselves may be set at any value within the longest interval a fixed length counter can time by counting clock pulses.
There are many applications that require control of the relative phase of a plurality of binary waveforms where the high and low states of the waveforms are not symmetrical, i.e. where the interval that a given waveform is low is not equal to the interval that the waveform is high, and there is some phase relationship to be controlled between waveforms. The term "binary waveform" thus means that each waveform may have only one of two levels at any given time, a high level such as +3V and a low level such as zero volts. The interval during which a waveform is high may be termed "active time". The term "multiphase waveform", as used herein, refers to a waveform in which the intervals of the active times can be selected. The rate at which the active times occur, as well as the intervals of the active times, must often be controlled in complex situations while at the same time controlling the relative phase between a plurality of binary multiphase waveforms.
An example of such a complex situation to be described by way of example in a specific embodiment, and not by way of limitation, is synthesizing multiphase waveforms for controlling the speed of a 2-phase induction motor. Each of two field windings is energized via an output power driver by two paired binary waveforms that have non-coincident active times. The output power driver employs solid state switches for switching the polarity of DC power applied. Since the load presented to the output power driver is inductive, there is a need for a dead time between phase active times in each of the two paired waveforms, i.e., a time during each half power cycle when neither one of two paired waveforms is high in order to protect the switches in the output power driver. This adds to the complexity of controlling the active times of one pair of waveforms in a predetermined phase relationship with respect to the other pair of waveforms for the second field winding of the motor. Consequently, in one set of four binary waveforms it is necessary to control the active times of one pair of waveforms with respect to each other and with respect to the other pair.